1. Field of the Invention
The present invention relates to the field of frequency stability measurement, and particularly to a method and apparatus for performing relative time interval error measurements for a multiplicity of inputs under microprocessor control.
2. Description of the Prior Art
Many microprocessor-based systems produce, during operation, a plurality of periodic signals, such as clock signals, which all have the same frequency but differ in phase from one another at different points in the system. In order for the system to operate properly, it is necessary to derive information about the difference in phase between one or more pairs of these periodic signals.
A number of techniques for directly measuring the stability of frequency sources and for deriving related numerical values for further processing have already been proposed and are in use. For example, methods are disclosed in: D. A. Howe, D. W. Allen and J. A. Barnes, "Properties of Signal Sources and Measurement Methods", National Institute of Standards and Technology Technical Note 1337, Characterization of Clocks and Oscillators, Washington, D.C., 1990, U.S. Government Printing Office, pp. TN-14 to TN-47. These methods include a beat frequency method, a dual mixer time difference method, a tight phase-lock loop method and a time difference method. All of these methods have the common characteristic that they utilize a conditioning circuit followed by a frequency counter or a time interval counter. The conditioning circuit serves to magnify phase differences between two periodic signals which are being compared and to square the edges of the periodic signals before delivery to the counter.
Frequency counters and time interval counters are similar devices. A time interval counter consists of a digital counter whose clock is driven by a free-running oscillator, together with a logic unit which begins counting in response to a signal state change at one input and halts counting in response to a signal change on another input. A frequency counter contains logic to start and stop counting in response to state changes in a signal applied to a single input. The resolution of each type of counter is limited to the period of the free-running clock, unless other conditioning takes place before inputs are supplied to the counter. Free-running clocks are usually limited to a frequency of several tens of megahertz for ease of implementation, although for dedicated equipment with special frequency multiplication circuitry, the free-running clock can run at a frequency close to 100 megahertz. Such dedicated equipment is described, for example, in Stanford Research Systems, Inc. Model SR620 Universal Time Interval Counter Operating Manual and Programming Reference, Revision 2.0, 1989, pp. 81-91. This limits the single-shot resolution of practical time interval counters used with a micro-processor in embedded frequency control applications to a few tens of nanoseconds, which is two coarse a resolution for precision frequency and time interval measurement.
The conditioning circuits employed for systems operating according to the beat frequency, dual mixer time difference and tight phase lock loop methods consist of analog filters, voltage-controlled oscillators and low-noise mixers. These circuits typically consist of a large number of discrete components, some of which are relatively expensive and bulky, and require several different power supply voltages. For systems operating according to the time difference method, the clock frequency is divided to very low frequency levels before presentation to the counter. This can create cycle ambiguity and significant dead time between measurements. These factors make it difficult to create a compact circuit which measures the relative time interval error of many different input signals with a high degree of precision.